Can't give a definitive answer based upon information given thus far. My guess is a possible incorrect Java installed. Java and LO must match bit-wise. 32-bit LO needs 32-bit Java. 64-bit LO needs 64-bit Java. From the menu, go to Tools-Options and under LibreOffice->Advanced you will see:
Insure Use a Java runtime environment is checked and you should (within a few seconds) have your Java appear in the pane if the 64-bit version is installed. If it appears, make sure it is selected (can have many Java versions installed). If nothing appears, please install the 64-bit version from Oracle (OpenJDK should also work).